Batteries used in stand alone power supply systems are commonly lead-acid batteries. However, lead-acid batteries have limitations in terms of performance and environmental safety. Typical lead-acid batteries often have very short lifetimes in hot climate conditions, especially when they are occasionally fully discharged. Lead-acid batteries are also environmentally hazardous, since lead is a major component of lead-acid batteries and can cause serious environmental problems during manufacturing and disposal.
Flowing electrolyte batteries, such as zinc-bromine batteries, zinc-chlorine batteries, and vanadium flow batteries, offer a potential to overcome the above mentioned limitations of lead-acid batteries. In particular, the useful lifetime of flowing electrolyte batteries is not affected by deep discharge applications, and the energy to weight ratio of flowing electrolyte batteries is up to six times higher than that of lead-acid batteries.
However, manufacturing flowing electrolyte batteries can be more difficult than manufacturing lead-acid batteries. A flowing electrolyte battery, like a lead acid battery, comprises a stack of cells to produce a certain voltage higher than that of individual cells. But unlike a lead acid battery, cells in a flowing electrolyte battery are hydraulically connected through an electrolyte circulation path. This can be problematic as shunt currents can flow through the electrolyte circulation path from one series-connected cell to another causing energy losses and imbalances in the individual charge states of the cells. To prevent or reduce such shunt currents, flowing electrolyte batteries define sufficiently long electrolyte circulation paths between cells, thereby increasing electrical resistance between cells.
Another problem of flowing electrolyte batteries is a need for a uniform electrolyte flow rate in each cell in order to supply chemicals evenly inside the cells. To achieve a uniform flow rate through the cells, flowing electrolyte batteries define complex flow distribution zones. However, because electrolyte often has an oily and gaseous multiphase nature, and because of structural constraints on the cells, uniform flow rates are often not achieved.
Referring to FIG. 1, a flow diagram illustrates a basic zinc-bromine flowing electrolyte battery 100, as known according to the prior art. The zinc-bromine battery 100 includes a negative circulation path 105 and an independent positive circulation path 110. The negative circulation path 105 contains zinc ions as an active chemical, and the positive circulation path 110 contains bromine ions as an active chemical. The zinc-bromine battery 100 also comprises a negative electrolyte pump 115, a positive electrolyte pump 120, a negative electrolyte tank 125, and a positive electrolyte tank 130. To obtain high voltage, the zinc-bromine battery 100 further comprises a stack of cells connected in a bipolar arrangement. For example, a cell 135 comprises half cells 140, 145 including bipolar electrode plate 155 and a micro porous separator plate 165. The zinc-bromine battery 100 then has a positive polarity end at a collector electrode plate 160, and a negative polarity end at a collector electrode plate 150.
A chemical reaction in a positive half cell, such as the half cell 145, during charging can be described according to the following equation:2Br−→2Br+2e−  Eq. 1Bromine is thus formed in half cells in hydraulic communication with the positive circulation path 110 and is then stored in the positive electrolyte tank 130. A chemical reaction in a negative half cell, such as the half cell 140, during charging can be described according to the following equation:Zn2++2e−→Zn  Eq. 2A metallic zinc layer 170 is thus formed on the collector electrode plate 150 in contact with the negative circulation path 105. Chemical reactions in the half cells 140, 145 during discharging are then the reverse of Eq. 1 and Eq. 2.
The prior art discloses various approaches for creating flow distribution zones that obtain uniform flow rates, and for creating substantially long circulation paths between cells in a cell stack of a flowing electrolyte battery. One approach defines coiled capillary tubes inside external manifolds that supply electrolyte to a cell stack. The coiled capillary tubes are connected to flow distribution zones defined in the cells via an array of elastomer connection tubes. Each cell has multiple inlets and outlets, and thus each external manifold has to be connected to the cell stack using a delicate connection apparatus comprising an array of elastomer connection tubes. A typical 54-cell stack requires 216 elastomer connection tubes. Such a delicate connection apparatus is not only difficult to manufacture, but is also prone to damage during assembly and use.
Another approach uses long circulation paths and flow distribution zones defined within cells. That reduces a number of external connection points. However, each cell in a cell stack then has to be welded internally to ensure that electrolyte does not leak out of a circulation path. A typical 60-cell stack may therefore have only 8 inlets/outlets, but it may have 121 critical external and internal weld seams.
Referring to FIG. 2, a diagram illustrates a perspective view of a cell stack 200 for a flowing electrolyte battery, as known according to the prior art. Cells in the cell stack 200 are connected to external manifolds 205 via an array of elastomer connection tubes 210. The cell stack 200 has 10 critical welding seams: a top welding seam 215, a bottom welding seam 216, four inlet/outlet tube sealing welding seams 225, and four manifold-tube welding seams 230.
Referring to FIG. 3, a diagram illustrates the supply of electrolyte to cells in the cell stack 200, as known according to the prior art. A coiled capillary tube 305 is placed in an external manifold 205 and connected to a flow distribution zone 310 defined in an electrode plate 315 via an elastomer connection tube 210.
Referring to FIG. 4, a diagram illustrates uneven electrolyte flow distribution along the electrode plate 315 of the cell stack 200, as known according to the prior art. Long arrows 400 indicate significant electrolyte flow rates across ends of the electrode plate 315; whereas short arrows 405 indicate reduced electrolyte flow rates across a middle section of the electrode plate 315. An over supply of electrolyte to any section of the electrode will cause a reduction in the efficiency of the battery. An under supply of electrolyte to any section of the electrode can permit dendrite formation which may lead to permanent damage to the separator and shorting between cells.
There is therefore a need to overcome or alleviate many of the above discussed problems associated with flowing electrolyte batteries of the prior art.